1. Field of the Invention
This invention relates to a method of manufacturing a grain-oriented silicon steel sheet having excellent magnetic properties in the rolling direction. More particularly, it relates to a method of heating a silion steel slab as a material for the silicon steel sheet which advantageously solves troubles attendant upon a necessary high-temperature heating.
2. Description of the Prior Art
As is well known, the grain-oriented silicon steel sheet is composed of secondary recrystallization grains wherein the (110) plane is oriented in the sheet surface and the [001] axis is oriented in the rolling direction. Therefore, it is important to selectively grow crystal grains of such an orientation during the final high temperature box annealing. For this purpose, it is necessary that so-called inhibitors such as MnS, MnSe, AlN and the like disperse uniformly and suppress the growth of crystal grains other than the secondary crystal grains so as to selectively grow the secondary recrystallization grain of (110)[001] orientation prior to the finish annealing.
It has already been well-known that the above dispersion form is controlled by once dissolving the precipitated inhibitors to a solid solution during the heating of the slab prior to the hot rolling and then subjecting the slab to a hot rolling according to a proper cooling pattern.
For this purpose, the heating of the slab is usually performed at an elevated temperature above 1,300.degree. C. Particularly, it is common to heat the outer surface of the slab at a temperature of not less than 1,350.degree. C. in order to sufficiently conduct the heating to the central portion of the slab. In this case, a greater amount of molten scale produced not only impairs the operability of the heating furnace but also raises many troubles such as surface defects due to oxidation of grain boundary, occurrence of edge cracks and the like.
In order to solve the above drawbacks, there have been proposed various methods wherein the heating is carried out at a low temperature by varying the amount of ingredients contained in the steel. For instance, Japanese Patent laid open No. 58-100,627 discloses that the amount of inhibitor and the C content in the steel material are decreased to lower the slab reheating temperature, and Japanese Patent Laid open No. 50-160,120 discloses that the dissolution product of Mn content.times.S content in the steel material is decreased to lower the solid solution limit of MnS for performing the low-temperature heating.
However, all of these methods can not act as a complete solution and have not yet been adopted in an industrial scale because the reduction of the inhibitor amount does not stabilize the magnetic properties. Also, the decrease of Mn content frequently produces the edge cracking and surface defect even after carrying out the low-temperature heating of the slab.
On the other hand, the inventors have found that when the conventional slab heating furnace of gas-firing type is used together with a slab induction heating furnace to reduce the temperature difference between the slab surface temperature and the slab center temperature in a high-temperature range, various troubles usually produced by heating the slab surface at an excessively high temperature can advantageously be solved and also the energy-saving and quality can be improved remarkably.
As the method of applying the induction heating to the slab, there are some reports as disclosed, for example, in Japanese Patent Application Publication No. 44-15,047, No. 52-47,179 and the like. The former report is concerned with the control of optimum energy in the multiple heating with a gas annealing furnace, while the latter report discloses a method of preventing the temperature drop at the slab end in the induction heating.
In addition, there are known some proposals relating to the slab induction heating furnace. Among them, however, only the proposal disclosed in Japanes Patent Application Publication No. 47-14,627 is applied to the heating of the slab for the grain-oriented silicon steel, wherein the slab for the grain-oriented silicon steel is heated at 1,250.degree.14 1,300.degree. C. in a gravity charging furnace (or a heating furnace of pusher type) and subsequently heated at a higher temperature of 1,350.degree.-1,400.degree. C. by an electrical means such as induction heating or resistance heating to improve the magnetic properties. In this case, the induction heating of the slab at the high temperature of 1,350.degree.-1,400.degree. C. without controlling the atmosphere is effective for the improvement of magnetic properties, but produces a fairly large amount of slag on the surface of the slab even for a short heating time, resulting in not only the impairing of the furnace operation but also the occurrence of surface defects. Further, when the slab containing no carbon of more than a given amount is heated till the above heating temperature reaches to the center of the slab, crystal grains in the slab become coarse to leave coarse extended grains in the hot rolled sheet, which then turns into fine grain streaks in the finished product to cause the local degradation of magnetic properties.